Control logic for fuel controls on APUs

ABSTRACT

Conventional auxiliary power units (APUs) may experience over-temperature shutdowns when attempting to start them at high altitudes. Further, such conventional APUs may also experience overspeed conditions when a generator load is removed during on-speed operations. A fuel control logic that controls the fuel flow cutback below the minimum blowout fuel schedule is provided. A temperature trim loop measures engine temperature to determine the onset of a possible over-temperature condition. The fuel flow may then be trimmed accordingly to correct this over-temperature onset. Further, when the onset of an overspeed condition is detected, such as when a generator load is removed, the fuel flow may be trimmed accordingly to correct this overspeed onset. The fuel control logic allows the control to find the individual minimum fuel flow for each fuel control without risking blowout of the APU itself.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 10/781,154 filed on Feb.17, 2004.

BACKGROUND OF THE INVENTION

The present invention relates generally to control logic for fuelcontrols on an auxiliary power unit (APU) and, in particular, to acontrol logic that allows the electronic control to command fuel flowsbelow the programmed lean blowout limit when certain conditions aretrue.

In addition to their traditional propulsion functions, gas turbineengines are often used as APUs to supply mechanical, electrical and/orpneumatic power to a wide variety of aircraft systems. For example, theAPU can be used to start the main engines, supply compressed air to theaircraft's environmental control system, or provide electrical power.Historically, APUs have only been operated when the aircraft was on theground.

Recent developments in aircraft design have witnessed the advent of twinengine aircraft capable of long distance, transoceanic flights. Adisadvantage to the twin engine design is that when a main engineexperiences an inflight shutdown, the enormous burden of supplying theaircraft with power falls on the sole, remaining engine. Early on in thedevelopment of these aircraft, it was recognized that they would need anadditional source of power while inflight. To meet this need, it wasproposed to start and operate the APU inflight.

A gas turbine APU includes in flow series arrangement a compressor, acombustor, a turbine and a shaft coupling the turbine to the compressor.During a normal, sea level start, a starter motor applies a startingtorque to the APU's shaft. As the shaft starts to rotate, air isinducted into the compressor, compressed and then discharged in thecombustor. Concurrently, the APU's fuel control system feeds fuel intothe combustor in accordance with a predetermined fuel schedule toprecisely maintain the proper fuel to air ration in the combustor. At arotational speed of about 10 to 20 percent of the APU's operating speed,the condition in the combustor becomes such that the fuel/air mixturecan be ignited. This condition is generally referred to as light-off.Should the fuel to air ratio be either too rich or too lean, light-offwill not occur and the APU will experience a hung start. Afterlight-off, the start motor torque is augmented by torque from the APU'sturbine. At about 50 percent of operating speed, the start motor is shutoff and the APU becomes self-sustaining and accelerates itself tooperating speed.

To start an APU at high altitude (e.g., 40,000 feet) after the APU hasbecome cold soaked by continuous exposure to cold ambient temperatures(e.g., −70F.) is a much more difficult task for the APU's fuel controlsystem. The cold temperature increases the APU's drag necessitatinggreater starting torque. Further, the cold fuel poorly atomized. Pooratomization combined with low air density makes it both difficult toprecisely obtain the necessary fuel to air ratio to accomplishlight-off, and to provide a sufficient fuel flow rate to the combustorto prevent blowout while not providing too high a fuel flow rate whichmay result in excessive turbine inlet temperatures.

Conventional fuel control logic has two significant problems. First, asmentioned above, starting APUs at high altitudes often result inover-temperature shutdowns due to tolerances in the fuel control duringlow fuel flows. These tolerances may include the difference between theactual fuel flow vs the milliamp (ma) command fuel flow based upon theconventional fuel control logic. When an over-temperature condition isdetected, the fuel supply is cut back. However, conventional fuelcontrol logic limits the fuel flow temperature cutback to a minimumcommand to prevent blowout on a nominal fuel control. In other words,there is a pre-programmed lean limit to the minimum fuel flow that mayoccur. This pre-programmed lean limit is determined at a level to avoidblowout of the engine.

Low fuel flows may be difficult to accurately measure and, therefore,conventional fuel control logic may require the use of a fuel flowfeedback mechanism to calibrate the commanded fuel flows. However,degradation in these fuel feedback mechanisms as well as other enginetolerances often has an effect on the true lean stability limit, whichmay be lower than the pre-programmed lean limit. Optionally, the fuelflow at low flows may be measured to tighter standards. However, both ofthese approaches may result in a significant cost impact to the systemdesign.

A second problem with conventional fuel control logic occurs duringon-speed operation of APUs (constant speed) at high altitudes. Here,tolerances in the fuel control during low fuel flows may cause the speedof the APU to react slowly to unloading of electrical loads. The fuelcontrols limit the fuel flow cutback to a minimum command to preventblowout on a nominal fuel control. However, engine overspeed may occurbecause the fuel flow is required to be at or above a minimum,preprogrammed fuel flow. As with solutions to the first problem, lowfuel flows may be difficult to accurately measure and, therefore,conventional fuel control logic may require the use of a fuel flowfeedback mechanism to calibrate the commanded fuel flows. Optionally,the fuel flow at low flows may be measured to tighter standards.However, both of these approaches may result in a significant costimpact to the system design.

U.S. Pat. Nos. 5,274,996 and 5,303,541, issued to Goff et al., describeusing a closed loop system on measured fuel flow to more accuratelycontrol fuel flow and improve starting reliability. The commanded fuelflow may be trimmed until it matches measured fuel flow. The APUs ofGoff, however, may experience fuel flow meter failures and fueltolerance problems, causing failures of the APU to start at highaltitudes.

U.S. Pat. No. 4,128,995, issued to Toot, discloses a method andapparatus for stabilizing an augmenter system. More specifically, thepatent discloses stabilizing a turbofan at high speed, high altitudeflight conditions by reducing the maximum augmenter fuel/air ration inresponse to certain pressure and temperature conditions. The Toot patentspecifically addresses a rich stability problem within the combustor.The reference does not discuss the issues of lean stability and minimumfuel flow tolerances.

As can be seen, there is a need for an improved fuel control logic thatwill allow the electronic control to command fuel flows below thepre-programmed lean blowout limit when certain conditions are true, thusavoiding overspeed and over-temperature conditions.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method for correcting and/orpreventing an over-temperature condition in an auxiliary power unitcomprises measuring an exhaust gas temperature of the auxiliary powerunit; comparing the exhaust gas temperature to a temperature trim limit;calculating a fuel flow trim value; subtracting the fuel flow trim valuefrom a starting fuel flow value to get a trimmed commanded fuel flowvalue; and delivering fuel to the auxiliary power unit at said trimmedcommanded fuel flow value.

In another aspect of the present invention, a method for correctingand/or preventing an overspeed condition in an auxiliary power unitcomprises measuring the speed of the auxiliary power unit; comparing thespeed to a predetermined speed reference point to determine a speederror; calculating a fuel flow trim value from the speed error;subtracting the fuel flow trim value from a commanded fuel flow value toget a trimmed commanded fuel flow value; and delivering fuel to theauxiliary power unit at said trimmed commanded fuel flow value.

In yet another aspect of the present invention, a method for correctingand/or preventing an overspeed condition during on-speed operation of anauxiliary power unit in an aircraft comprises measuring the speed of theauxiliary power unit; comparing the speed to a predetermined speedreference point to determine a speed error; calculating a fuel flow trimvalue from the speed error; subtracting the fuel flow trim value from acommanded fuel flow value to get a trimmed commanded fuel flow value;providing an upper limit on said the fuel flow trim value at apredetermined maximum fuel schedule; determining a blowout preventionrate; comparing the trimmed commanded fuel flow value to a predeterminedminimum fuel schedule to determine a possibility of blowout at thetrimmed commanded fuel flow value; delivering fuel to the auxiliarypower unit at the blowout prevention rate when the possibility ofblowout is present, thereby preventing blowout of the auxiliary powerunit; and delivering fuel to the auxiliary power unit at said trimmedcommanded fuel flow value when the possibility of blowout is notpresent.

In a further aspect of the present invention, a method for preventingand/or correcting undesired operating conditions of an auxiliary powerunit comprises calculating an engine starting trimmed commanded fuelflow value by measuring an exhaust gas temperature of the auxiliarypower unit; comparing the exhaust gas temperature to a temperature trimlimit; calculating a fuel flow trim value; subtracting the fuel flowtrim value from a starting fuel flow value to get the engine startingtrimmed commanded fuel flow value; starting the auxiliary power unitwith a fuel flow rate at the engine starting trimmed commanded fuel flowvalue; calculating an on-speed trimmed commanded fuel flow value bymeasuring the speed of the auxiliary power unit; comparing the speed toa predetermined speed reference point to determine a speed error;calculating an on-speed fuel flow trim value from the speed error;subtracting the on-speed fuel flow trim value from a commanded fuel flowvalue to get the on-speed trimmed commanded fuel flow value; continuingthe running of the auxiliary power unit with a fuel flow rate at theon-speed trimmed commanded fuel flow value.

In still a further aspect of the present invention, a fuel control logicfor an auxiliary power unit comprises an over-temperature preventionand/or correcting mechanism to prevent an over-temperature conditionfrom occurring during starting of the auxiliary power unit; theover-temperature prevention and/or correcting mechanism operating bymeasuring an exhaust gas temperature of the auxiliary power unit;comparing the exhaust gas temperature to a temperature trim limit;calculating a fuel flow trim value; subtracting the fuel flow trim valuefrom a starting fuel flow value to get a trimmed commanded fuel flowvalue; and delivering fuel to the auxiliary power unit at said trimmedcommanded fuel flow value.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of an exemplary gas turbineauxiliary power unit having the fuel control logic of the presentinvention;

FIG. 2 is a schematic diagram showing the starting command fuel flowlogic according to the present invention;

FIG. 3 is a series of graphs showing various engine statistics over timeduring APU start according to the present invention;

FIG. 4 is a schematic diagram showing the APU on-speed commanded fuelflow logic according to the present invention; and

FIGS. 5A, 5B and 5C are a series of graph showing various enginestatistics over time during on-speed conditions according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

Briefly, the present invention provides a method and apparatus forcontrolling a minimum fuel flow below a minimum blowout schedule, thatis, below a minimum fuel flow predetermined to cause engine blowout. Bydoing so, the present invention provides a solution to the problem ofover-temperature shutdowns when starting APUs at high altitudes, as wellas to the problem of overspeed operation of APUs at high altitudes whenelectric loads are unloaded.

Conventional fuel control methods may adjust the fuel flow and/or thefuel/air mixture to avoid over-temperature and overspeed conditions.However, low fuel flows may be difficult to accurately measure and,therefore, conventional fuel control logic may require the use of a fuelflow feedback mechanism to calibrate the commanded fuel flows.Optionally, the fuel flow at low flows may be measured to tighterstandards. However, both of these approaches may result in a significantcost impact to the system design.

In contrast, the fuel control logic of the present invention provides amethod for allowing a fuel cutback to below minimum, predeterminedblowout fuel schedules while maintaining the operation of the APU. Thelogic allows the control to find the individual minimum fuel flow foreach fuel control without risking blowout on the APU itself. The fuelcontrol logic of the present invention is useful on any gas turbineengine, especially jet engine APUs.

Referring to FIG. 1, one form of an APU to which the present inventionrelates is generally denoted by the reference numeral 10. APU 10 mayinclude, in flow series arrangement, an air inlet 14, a compressor 16, ableed port 18 for providing compressed air to the aircraft, a combustor20 having a primary fuel nozzle 22 and a secondary fuel nozzle 24, aturbine 26, and a gas exhaust 28. Of the two nozzles 22 and 24, onlyprimary nozzle 22 may operate during the initial stages of a startup.Compressor 16 and turbine 26 may be mounted for rotation on a shaft 30which extends to a gearbox 32.

A fuel control unit 40 may be drivingly mounted to gearbox 32 in fluidcommunication with a fuel source (not shown) aboard the aircraft. In anembodiment, fuel control unit 40 may be a single stage,electromechanical fuel-metering valve of the type which is well known inthe art. Fuel control unit 40 may include an electrically operatedtorque motor 42 which has a known and repeatable relationship with apower signal from an electronic control unit (ECU) 80 which may beanalog or digital. Motor 42 may be directly coupled to a metering valve(not shown), and controls the valve position so that a known flow areain the metering valve corresponds to a known power signal from ECU 80. Aregulating valve (not shown) may maintain a constant pressure dropacross the metering valve so that the metered flow is a direct functionof the power signal. Fuel control unit 40 may receive fuel from the fuelsource via a high-pressure fuel pump, and discharge a metered fuel flowthrough a conduit 46 to a flow divider 50. Other accessories (not shown)such as start motors, electrical generators and pumps can also bemounted to gearbox 32.

Flow divider 50 may include a filter 52 through which metered fuel isreceived via conduit 46 and then may be passed by a temperature sensor54 which is electrically coupled to ECU 80. Downstream of sensor 54, theflow of fuel within divider 50 may be broken into a primary flow path 56and a secondary flow path 66.

Fuel entering primary flow path 56 may flow through a conventional flowmeter 58. Flow meter 58 may measure the rate of flow passingtherethrough and convert this measurement to an electrical signal whichis transmitted to ECU 80. After passing through flow meter 58, thestream of fuel may enter a conduit 60 which leads to primary fuel nozzle22. Disposed between flow meter 58 and conduit 60 may be a primary drainvalve 62 which, when open, places conduit 60 in fluid communication witha drain port 64 and, when closed, with conduit 46.

Fuel may only enter secondary flow path 66 when its pressure issufficient to open a start sequence valve 68. When valve 68 is open,fuel may flow past a secondary drain valve 70 similar to primary drainvalve 62 and into a conduit 72 that leads to secondary fuel nozzle 24.When valve 70 is closed, conduit 72 may be placed in fluid communicationwith drain port 64.

Prior to starting APU 10, start sequence valve 68 may be closed anddrain valves 62 and 70 may be opened. Upon receiving a signal from ECU80, fuel control unit 40 may meter fuel from the fuel source to divider50 and through primary flow path 56, wherein primary drain valve 62 mayclose and fuel may flow to primary fuel nozzles 22. During this time thefuel flow in primary flow path 56 may be measured by flow meter 58.After light-off, the fuel pressure may increase until start sequencevalve 68 opens, and secondary drain valve 70 may close, and fuel maystart flowing to secondary nozzles 24.

While the above APU 10 has been described using flow meter 58 on primaryflow path 56, the fuel flow may also be measured on the entire flow. Inother words, a second flow meter (not shown) may be used on secondaryflow path 66 and a combined fuel flow measurement, from both primaryflow path 56 and secondary flow path 66, may be taken.

Fuel control logics 100 and 200, illustrated in FIGS. 2 and 4,respectively, may be incorporated within ECU 80. These fuel controllogics may provide for both starting commanded fuel flow and APUon-speed commanded fuel flow.

Referring to FIG. 2, there is shown fuel control logic 100 for an enginestarting commanded fuel flow WFA. Function block 102 may take input ofvarious parameters—such as APU speed, airflow pressure andtemperature—to determine a calculated starting fuel flow WFA_CMD.Function block 104 may verify that calculated starting fuel flow WFA_CMDis between a minimum fuel schedule and a maximum fuel schedule. If so,function block 104 may output the calculated starting fuel flow WFA_CMD.If not, function block 104 may output either the maximum fuel scheduleor the minimum fuel schedule, whichever is closer to the calculatedstarting fuel flow WFA_CMD.

A temperature trim logic 105 may compare engine exhaust gas temperatureEGT to a predetermined temperature trim limit to get a temperature errorTMPERR. The trim error TRIM_LL_ERR may then be executed through aproportional (with proportional controller KP) plus integral (withintegral controller KI/S) feedback routine. The output of this routine,indicated at arrow 106, may pass into function block 108. If output 106is greater than zero lb/hour, function block 108 may output a trim fuelflow WFA_TRIM the same as output 106. If output 106 is not greater thanzero, trim fuel flow WFA_TRIM is zero.

Trim fuel flow WFA_TRIM may then be subtracted from the output offunction block 104 to give an output 110. Function block 112 may compareoutput 110 to a flow rate of zero and output the higher flow rate as acommanded fuel flow WFA. This commanded fuel flow WFA may then used forengine starting.

In summary, the present invention may use temperature trim logic 105during engine starting to allow the fuel to trim back below the leanblowout schedule, if necessary. By measuring the engine exhaust gastemperature EGT, a trimmed fuel flow WFA_TRIM may be determined to lowerthe calculated fuel flow below the minimum fuel schedule, thus avoidingan engine over-temperature condition.

Referring to FIG. 3, and for purposes of illustrating the presentinvention, there is shown a series of graphs depicting various enginestatistics over time during APU start. Trace A shows the APU speed as apercentage of maximum speed over time; trace B shows the exhaust gastemperature EGT of the APU (° F.); trace C shows the primary fuelmanifold delta pressure (psid) from which actual fuel flow can becalculated; and trace D shows the fuel control torque motor current(mA).

A first time point 210 shows where the fuel was introduced into thecombustor, ignition was achieved and the exhaust gas temperature beganto rise. The speed of the APU engine was also increasing at this time,being driven mostly by a starter motor as described above and in FIG. 1.At second time point 220, the temperature trim logic began to output atrim fuel flow WFA_TRIM that is greater than zero, thus causing thecommanded fuel flow WFA to decrease below the minimum fuel schedule. Asa result, as time goes on, past second time point 220, the exhaust gastemperature EGT decreased. At third time point 230, it can beappreciated that the exhaust gas temperature EGT was no longer in riskof resulting in an engine over-temperature condition and, therefore, thetemperature trim logic no longer adjusted the commanded fuel flow WFA.Referring now to FIG. 4, there is shown a schematic diagram of the APUon-speed commanded fuel flow logic 300 according to the presentinvention. One purpose of this aspect of the present invention is toprevent overspeed conditions when the load upon the APU changes, such aswhen an electrical load is removed from the system.

A predetermined APU speed reference point may be programmed into thecommanded fuel flow logic 300. The actual APU speed may then compared tothe speed reference point to give SPEED_ERROR. The SPEED_ERROR may beexecuted through a proportional (with proportional controller KP) plusintegral (with integral controller KI/S) feedback routine. The output ofthis routine 302, called the commanded fuel flow WFG_CMD, may pass intofunction block 310, which compared the commanded fuel flow WFG_CMD tothe maximum fuel schedule. Function block 310 may then output the lowerof these two inputs, called the commanded fuel flow upper limitWFGOV_UPLIM. A function block 308 may compare the commanded fuel flowupper limit WFGOV_UPLIM to a minimum calculated fuel flow 306 calculatedby subset 304 (described below). Function block 308 may then output thecommanded fuel flow during on-speed APU operation WFGOV, which isWFGOV_UPLIM restrained by the rate limits of fuel reduction 306.

Subset 304 may be used to determine the allowed fuel flow reduction rate306 so as to prevent blowout during on-speed APU operation. Thecommanded fuel flow upper limit WFGOV_UPLIM may be compared with theminimum fuel schedule to determine if a rate limited fuel reduction isnecessary. If no rate limited fuel reduction is determined necessary toprevent engine blowout, then the fuel reduction rate 306 may provide aninput suggesting NO_LIMIT into function block 308. However, as shown inFIG. 4, if the comparison of the commanded fuel flow upper limitWFGOV_UPLIM with the minimum fuel schedule suggests that blowout mayoccur, the value PREVENT_BLOWOUT_RATE may be inputted into functionblock 308 as the fuel rate limiter to prevent engine blowout.

Referring now to FIGS. 5A, 5B and 5C, and for purposes of illustratingthe present invention, there are shown a series of graph depictingvarious engine statistics over time during on-speed conditions using theon-speed commanded fuel flow logic 300 of the present invention. FIG. 5Ashows the APU speed as a percentage of maximum speed over time; FIG. 5Bshows the generator load (kVA); and FIG. 5C shows the fuel control unittorque motor current (mA).

At a first time point 410, the APU was operating in a normal on-speedoperating condition, with the APU speed at 100%, the generator load atabout 91 kVA, and the fuel control unit torque motor current at about 38mA. At a second time point 420, the load was removed from the generator,causing the APU speed to increase above 100%. This caused the on-speedcommanded fuel flow logic 300 to employ, decreasing the fuel controlunit torque motor current appropriately below its minimum fuel scheduleto rapidly (in this example, in about three seconds) bring the APUoverspeed condition back to the 100% speed point. At a third time point430, the fuel control unit torque motor current returns to its requiredto run fuel schedule to operate the APU at 100% speed with no generatorload.

It should be understood, of course, that the foregoing relates topreferred embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1. A fuel control logic for an auxiliary power unit comprising: anover-temperature prevention and/or correcting mechanism to prevent anover-temperature condition from occurring during starting of theauxiliary power unit; the over-temperature prevention and/or correctingmechanism operating by measuring an exhaust gas temperature of theauxiliary power unit, comparing the exhaust gas temperature to atemperature trim limit, calculating a fuel flow trim value, subtractingthe fuel flow trim value from a starting fuel flow value to get atrimmed commanded fuel flow value, and delivering fuel to the auxiliarypower unit at said trimmed commanded fuel flow value.
 2. The fuelcontrol logic according to claim 1, wherein the over-temperatureprevention and/or correcting mechanism further operates by determiningthe starting fuel flow value based upon speed, air pressure andtemperature of the auxiliary power unit; and providing an upper limit ata predetermined maximum fuel schedule and a lower limit at apredetermined minimum fuel schedule for the starting fuel flow value. 3.The fuel control logic according to claim 1, wherein the trimmedcommanded fuel flow may be zero.
 4. The fuel control logic according toclaim 1, further comprising: an overspeed prevention and/or correctingmechanism to prevent an overspeed condition from occurring duringon-speed operation of the auxiliary power unit; the overspeed preventionand/or correcting mechanism operating by measuring the speed of theauxiliary power unit, comparing the speed to a predetermined speedreference point to determine a speed error, calculating an on-speed fuelflow trim value from the speed error, subtracting the on-speed fuel flowtrim value from an on-speed commanded fuel flow value to get an on-speedtrimmed commanded fuel flow value, and delivering fuel to the auxiliarypower unit at said on-speed trimmed commanded fuel flow value.
 5. Thefuel control logic according to claim 4, wherein the overspeedprevention and/or correcting mechanism further operates by determining ablowout prevention rate, comparing the on-speed trimmed commanded fuelflow value to a predetermined minimum fuel schedule to determine apossibility of blowout at the on-speed trimmed commanded fuel flowvalue, and providing fuel to the auxiliary power unit at the blowoutprevention rate when the possibility of blowout is present, therebypreventing blowout of the auxiliary power unit.
 6. The fuel controllogic according to claim 5, wherein the overspeed prevention and/orcorrecting mechanism further operates by providing an upper limit on theon-speed fuel flow trim value at a predetermined maximum fuel schedule.7. An auxiliary power unit for an aircraft comprising: an air inlet; acompressor in flow arrangement with the air inlet; a bleed port forproviding compressed air to the aircraft; a combustor having at leastone fuel nozzle; a turbine in flow arrangement with a gas exhaust; afuel control logic having an over-temperature prevention and/orcorrecting mechanism to prevent an over-temperature condition fromoccurring during starting of the auxiliary power unit; and theover-temperature prevention and/or correcting mechanism operating bymeasuring an exhaust gas temperature of the auxiliary power unit,comparing the exhaust gas temperature to a temperature trim limit,calculating a fuel flow trim value, subtracting the fuel flow trim valuefrom a starting fuel flow value to get a trimmed commanded fuel flowvalue, and delivering fuel to the auxiliary power unit at said trimmedcommanded fuel flow value.
 8. The auxiliary power unit according toclaim 7, further comprising: an overspeed prevention and/or correctingmechanism to prevent an overspeed condition from occurring duringon-speed operation of the auxiliary power unit; the overspeed preventionand/or correcting mechanism operating by measuring the speed of theauxiliary power unit, comparing the speed to a predetermined speedreference point to determine a speed error, calculating an on-speed fuelflow trim value from the speed error, subtracting the on-speed fuel flowtrim value from an on-speed commanded fuel flow value to get an on-speedtrimmed commanded fuel flow value, and delivering fuel to the auxiliarypower unit at said on-speed trimmed commanded fuel flow value.